In search of a second implementation

For {encoder, decoder, format} it seems to me we can use the reference impl
as implementation 1 and FontTools as implementation 2. Currently they
wouldn't both pass all tests but we have a clear path to make it so.

For User Agent, the reference impl can be implementation 1 but to call
FontTools a User Agent seems a bit of a stretch.

So where does UA#2 come from?